1. Field of the Invention
This invention relates to optical communication and, in particular, to tap couplers suitable for sampling signals within optical devices, waveguides or fiber arrays.
2. Description of the Related Art
The control and monitoring of optical devices, networks and communication links often require extracting a small fraction of light from an optical channel. The extracted light is detected and can be used to monitor the channel for status information. It can also be used for active adjustment or equalization of the power in one channel with the power levels in other channels via a variable attenuator or active switching devices with closed loop feedback control. For example, reconfigurable OADMs (optical add-drop modules) might use power monitoring for proper optimization of express, add and drop channel throughputs and for overall power balancing within appropriate channels.
A conventional method for extracting a small fraction of light from a single mode fiber uses a fused biconical taper tap coupler. FIG. 9 shows an array of fibers in a fiber optic ribbon 102, where the fibers 102a run parallel to one another with a fixed distance between adjacent fibers (channels). According to the conventional technique, the fibers 102a are separated away from the tight configuration within the fiber ribbon 102 to allow for connecting and routing of the individual fibers to the fused biconical taper tap couplers 104. Downstream from the couplers, the output fibers 106a and tapped fibers 108a are regrouped into fiber ribbons 106 and 108, respectively. The fused biconical taper tap coupler is rugged and easy to implement for one or a few fibers. Four channels and four tap couplers are shown in this drawing. However, for fiber arrays and fiber array devices such as variable optical attenuators and reconfigurable OADMs, the channel count can approach and even exceed one hundred, requiring over one hundred tap couplers for channel monitoring. In these cases the implementation of biconical taper tap couplers becomes cumbersome and costly, as the couplers have a large volume and require considerable labor for assembly.
A V block assembly is a well-known tool that can be used to terminate the array of fibers within a fiber optic ribbon and to provide access to the optical signals within the individual fiber optic channels. A V block has a substrate made of silicon, glass, ceramic or other material. The fibers extending from the fiber optic ribbon are accommodated in a series of evenly-spaced grooves formed on the substrate, and are typically attached to the V block with an adhesive such as epoxy. The end surface of the V block where the optical fibers terminate is a polished flat surface, typically AR (anti-reflection) coated to maximize optical output, and typically not perpendicular to the optical axis of the fibers to suppress retro-reflection. The polished fiber ends are precisely registered with respect to one another within the V block.
A V block assembly may be used to couple light signals into and out of a variety of optical devices, including passive and active waveguide structures, such as AWGs (arrayed waveguide gratings) and optical switches, as well as non-waveguide structures such as detector arrays. For coupling light from the V block into a waveguide structure, the polished end surface of the V block is separated by a small uniform gap from a matching end surface of the waveguide structure and its supporting substrate. The gap is either filled with air or an optically transmissive epoxy. A typical gap width is less than 20 microns. Each fiber of the V block couples light into a corresponding waveguide across the gap. Coupling losses between the output fiber and the receiving waveguide are typically low (less than 0.1 dB) Similarly, optical signals can be coupled from a waveguide structure into a V block assembly, where light exits from the waveguide, crosses a small gap and enters the array of receiving optical fibers in the V block. A waveguide optical device may be used with both an input V block and an output V block, and the number of input and output channels may be different. For example, an AWG multiplexer may have a single input channel and a plurality of output channels.